The advent of the energy crises has brought about the desirability of operating gas turbine power-generating plants directly on the gaseous products produced by the burning of coal. Also, since high sulfur content coal is in plentiful supply, the burning of such coal in fluidized-bed combustors is most desirable because the coal is burned in the presence of limestone or dolomite so that the sulfur reacts with the dolomite to form calcium sulfate which is then easily removed with the ash.
It is well known that in fluidized-bed combustors for the burning of coal, as is exemplified in the British patent to Union Carbide Corp., No. 935,658, dated Sept. 4, 1963, the temperature in the combustor bed must be kept below the fusion point of the ash. Accordingly, it is common practice to provide heat exchange means in the combustor bed through which controlled amounts of cooling fluid flow effects maintenance of the bed temperatures within a specified range below the fusion point of the ash. The cooling fluid, as exemplified in the U.S. Patent to Jubb et al, U.S. Pat. No. 3,791,137, dated Feb. 12, 1974, may be helium or water or air as disclosed on page 79 of the report entitled "The Proceedings of the Fourth International Conference on Fluidized Bed Combustion" sponsored by ERDA and coordinated by the MITRE Corporation. One development in fluidized-bed combustion systems disclosed on page 325 of the aforesaid report calls for passage of about two-thirds (2/3) of the total compressed air generated through a tube bundle immersed in the bed for temperature control and the remaining one-third (1/3) used to fluidize and support combustion in the bed, the heated air passing from the bed into mixture with the discharged combustion gases. In this system the problem of carry-over of alkali metals, ash and other particulate material to the gas turbine is reduced since the heated compressed air is clean. Because of the relatively low heat transfer coefficient of air, the heat exchanger units must be relatively large in size to provide the necessary rate of heat transfer. Also, in such systems, if a rupture occurs in the tubes in the bed, escape of air into the bed will change the bed depth from the level for optimum efficiency and reduce the amount of clean air mixed with the discharged combustion products, all of which is detrimental to the efficiency of such systems. These and other disadvantages of presently known fluidized-bed combustion power systems are overcome by this invention.
It is, therefore, an object of this invention to provide a gas turbine power system utilizing a fluidized-bed combustor for burning coal which has improved heat transfer rates so that the heat exchangers may be of smaller size for the same system capacity.
It is another object of the present invention to provide a gas turbine power system utilizing a fluidized-bed combustor for burning coal wherein compressed air is heated by the heat in the fluidized bed without the attendant risk of leakage of such air into the bed and with negligible possibility of burn-out of heat transfer tubes immersed in the fluidized bed.
It is a further object of the present invention to provide a gas turbine power system having a fluidized-bed combustor for burning coal which is capable of operating at high temperature levels for optimum thermo-dynamic efficiency.